MICRODISCHARGES AS SOURCES OF PHOTONS, RADICALS AND THRUST*

Transcription

1 MICRODISCHARGES AS SOURCES OF PHOTONS, RADICALS AND THRUST* Ramesh Arakoni a) and Mark J. Kushner b) a) Dept. Aerospace Engineering b) Dept. Electrical and Computer Engineering Urbana, IL USA September 2004 * Work supported by the National Science Foundation and Air Force Research Labs. GEC04_01

2 AGENDA Introduction to microdischarge (MD) devices. Description of model. Scaling of the annular sandwich MD Visible and Excimer Emission Thrust Radicals Concluding remarks. GEC04_02

3 MICRODISCHARGE PLASMA SOURCES Microdischarges are plasma devices which leverage pd scaling to operate dc atmospheric glows 10s 100s µm in size V, a few ma Although similar to PDP cells, MDs are usually dc devices which largely rely on nonequilibrium beam components of the EED. Electrostatic nonequilibrium results from their small size. Debye lengths and cathode falls are commensurate with size of devices. Ref: Kurt Becker, GEC 2003 GEC04_03

4 DESCRIPTION OF MODEL To investigate scaling processes in microdischarge sources, nonpdpsim has been developed, a 2-dimensional model. Rectilinear or cylindrical unstructured mesh Implicit drift-diffusion-advection for charged species Navier-Stokes for neutral species Poisson s equation (volume, surface charge, material conduction) Circuit model Electron energy equation coupled with Boltzmann solution Monte Carlo beam electrons Optically thick radiation transport with photoionization Secondary electrons by impact, thermionics, photo-emission Surface chemistry GEC04_04

5 200 µm BASE CASE PARAMETERS Sloped dielectric (flow issues) Hole: 200 µm diameter at anode to 300 µm at cathode. Dielectric: 200 µm thick Anode/Cathode 100 µm thick Cylindrically symmetric 200 µm Argon, 250 Torr, 2 ma (set by adjusting ballast resistor) Meshing is critical ( dynamic range) Total nodes: 5424 Plasma nodes: 3693 GEC04_05

6 ELECTRIC POTENTIAL AND FIELDS Anode potential penetrates into lower plenum, producing hollowcathode-like structure. Geometrical enhancement and space charge produce fields approaching 100 kv/cm. GEC04_06 Electric Potential E/N (Electric Field/Gas Density) Max = 80 kv/cm

7 ELECTRON TEMPERATURE AND IONIZATION SOURCES In the bulk plasma, T e of 3.5 ev suggests positive column conditions. Large contributions to ionization occur from both bulk and beam electrons Electron Temperature Bulk Ionization Beam ionization GEC04_07

8 ELECTRON DENSITY Peak electron densities of >10 14 cm -3 are produced in the steady state. These high cw densities enable large rates of excitation of high lying electronic states. Electron density GEC04_08

9 THERMODYNAMIC PROPERTIES Current densities of 5-10 A/cm 2 and power of 10s-100 kw/cm 3 produce significant gas heating and rarefaction. Rarefaction increases range of secondary electrons. GEC04_09 Gas Temperature Relative Mass Density

10 ADVECTIVE FLOWFIELD Cataphoresis entrains gas, producing pumping action from above the plenum, through the hole to below the plenum. The jet experiences resistance in the stagnation zone below the plenum and recirculation results. Axial Gas Speed Flow Direction GEC04_10

11 VISIBLE AND UV EMISSION Visible emission is constrained to an annulus due to short lifetimes of states. UV emission from excimer is more distributed due to the large range of Ar(4s) metastable precursor. Ar(4p) Density (Visible Emission) GEC04_11 Ar 2 * Density (UV Emission)

12 Beam Ionization MD PROPERTIES vs PRESSURE Electron Density Decreasing pressure enables deeper penetration of beam electrons in spite of the lower cathode voltage. The result is more confinement at higher pressure and higher peak electron density. Ar, 2 ma 125 Torr 250 Torr 500 Torr GEC04_12

13 MD PROPERTIES vs PRESSURE: VISIBLE EMISSION Visible emission is significantly more extended at low pressure, penetrating far out the hole. Peak emission is greater at higher pressure due to confinement of beam component. 125 Torr 250 Torr 500 Torr GEC04_13

14 MD PROPERTIES vs PRESSURE: Ar(4s) DENSITY Excited state densities increase with increasing pressure due to higher stopping power of gas. Ref: Maria Cristina Penache, Thesis, 2002 GEC04_14 Ar, 2 ma

15 SENSITIVITY TO SHAPE: [e], AXIAL FLOW Straight Speed of (downward) axial flow produced by cataphoresis is > 50% higher in the less tapered MD. Higher current density, larger E/N, larger on-axis plasma density all contribute. Ar, 250 Torr, 2 ma GEC04_15 Tapered

16 MD SUSTAINED IN He/O 2 : ELECTRON, ION DENSITIES [e] [+ ions] [- ions] Large current densities and intrinsically high gas flow makes MDs ideal for reactant generators. Negative ions are dominated by O 2 - at pressures of 100s Torr. GEC04_16 He/O 2 =90/10, 125 Torr, 2 ma

17 MD SUSTAINED IN He/O 2 : RADICAL, EXCITED STATE DENSITIES Σ [O] [O 2 ( 1 )] [O 3 ] The range of O atoms is limited by recombination and ozone formation. O 2 ( 1 ) and O 3 are final products, having longer ranges. Cataphoresis induced flow preferentially ejects reactants downward. GEC04_17 He/O 2 =90/10, 125 Torr, 2 ma

18 MD SUSTAINED IN He/O 2 : FLOW PROPERTIES Optimization of MDs as radical sources will require careful attention to flow properties to maximize delivery of reactants. GEC04_18 He/O 2 =90/10, 125 Torr, 2 ma

19 CONCLUDING REMARKS Annular sandwich microdischarges have been computationally investigated. Ionization is largely dominated by beam components though bulk ionization in positive-column like regions also heavily contribute. Diffusive transport by long-lived metastables enable both visible (by multistep processes) and excimer emission far beyond electrodes. Cataphoresis produces ion pumping which can produce jets of gas through MD device. Use of MDs as radical generators will require careful optimization for dissociation and delivery (through gas dynamics). GEC04_19